(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of task queuing in order to reduce traffic jams and for controlling the priority in an Automatic Material Handling System (AMHS).
(2) Description of the Prior Art
For the fabrication of semiconductor devices, wafers form a basic component of such a fabrication process. In order to effectively manage a relatively large semiconductor foundry, such an operation is typically divided in separate functional or operational units or bays. Each bay is specifically designed and used for a particular operation that is provided within the functional scope and equipment capabilities of the bay. Since wafers form a basic component for the process of creating semiconductor devices, it stands to reason that wafers must be transported to the location where wafer undergo particular processing steps. The transport of wafers within or between bays is typically an automated operation, where the method of transport is arranged along lines of placing multiple wafer in wafer cassettes or wafer boats and this multiplicity of wafers is transported as a unit. The number of wafers contained in one wafer cassette may be about twenty-five wafers. Before the wafers are loaded into the wafer cassette, the storage of raw wafers is typically provided by a relatively large semi-automated cabinet, where entry and exit of the wafer into and from the cabinet is machine or operator controlled or by a combination thereof. Wafers that are placed inside the wafer cabinet are preferably first placed into the wafer cassettes, this in order to avoid unnecessary handling of the wafers at the time that the wafers are removed from the wafer cabinet.
Wafer cabinets are provided with entry and exit ports, through which the wafer cassettes with the therein-placed wafers proceed. It is further not unusual for empty wafer cassettes to be placed inside the wafer cabinet, from which these empty cassettes are retrieved on operator request.
All of these operations are a subset of moving product, of which wafers from a significant part, through a semiconductor foundry. Computers have long since found application and implementation in managing the logistics and traffic flow that underlies this flow of product. Computers and their ability to almost instantly react to either manually of automatically submitted requests for an operation or to automatically detect conditions in a processing environment are ideally suited for this type of application. In addition, computers are ideally suited for interfacing floor or logistics control systems with a host of other and related systems that equally apply to the ordering and finally shipping of semiconductor devices. In view of the numerous steps that must be performed before an ordered product can be delivered to a customer, steps of creating the product which implies moving the product through the foundry, of testing, of quality assurance, of cost analysis and of numerous other steps, and further in view of the fact that most of these steps are closely related, computers and the therewith associated and supporting computer software is ideally suited for this kind of application.
Since cost is an underlying motivator in any of the applications that relate to creating semiconductor devices, considerations of cost also weigh heavily as they relate to aspects of logistics or moving product through the semiconductor foundry. It stands to reason that such a process of logistics must be cost-effective, which further implies that the product must be moved without incurring delays and that priority assignment and modification thereof must be part of a logistics control system. Any deviation from this optimum product flow, such as an irregular flow of product (“traffic jam”) or moving product in an incorrect priority, can be expected to add cost to the process of logistics of the foundry, which will ultimately reflect in the cost of the product itself. The invention addresses these concerns and provides for a method of reducing traffic jams in the flow of product and for issues of priority assignment.
U.S. Pat. No. 6,308,107 to Conboy et al. discloses a real-time decision making system for the reduction of time delays in an AMHS. The system compares historical routing times with actual routing times to select alternative routings for material.
U.S. Pat. No. 6,108,585 to Ryan et al. describes a method for dispatching wafers in an AMHS. Probability distribution is used in the dispatch algorithm.
U.S. Pat. No. 6,035,245 to Conboy et al teaches an AMHS. A data process system is used to maintain historical records of material movement intervals. These records are used in an algorithm to achieve minimum movement times.